The present invention is related to the field of position encoders including diffractive optical position encoders.
It is well known in the art that the accuracy of a scale-based optical encoder is ultimately limited by the perfection of the scale itself. In diffractive optical encoders, various scale errors affect the position, period, straightness, and intensity profile of the interference fringes, which in turn affect the accuracy of the phase measurement performed by the encoder.
A typical approach for reducing the significance of scale errors is to perform a calibration measurement of the scale, either at the factory or in the field, and to apply the calibration to subsequent operational measurements. Usually the scale position is measured relative to an encoder sensor head while being translated on a stage whose position is independently sampled with a second, known good position measuring system (the “reference” system), such as an interferometer. In general, measurements are taken at intervals along the scale grating. Any differences between the position as measured by the encoder and the position as measured by the reference sensor are ascribed to the scale under test. Generally, these recorded differences are supplied to the user of the scale as a hard copy printout and/or as a digital file. Often the digital file can be incorporated as a look-up table (LUT) in an electronic processor used in conjunction with the encoder. The LUT is used to convert operational measurements based on the scale into more accurate compensated measurements.
Although the above compensation technique can be quite accurate, it has the drawback that the calibration is performed outside the system in which the encoder is used, and thus necessarily cannot capture errors that depend on the operating environment, such as temperature-related scale errors. Also, it is cumbersome or practically impossible to perform any re-calibration of the encoder once it is installed in an operating system, because of the need to connect it to a separate measuring system such as an interferometer.
An alternative prior art approach for compensating for scale errors is to apply a single, linear correction to the operational measurements. To the extent that the largest component of the error in the scale can be characterized by a linear function, this approach has proved valuable. However, one difficulty with this approach has been the need to apply a continuous linear correction in the form of a discontinuous staircase function. This problem was addressed by Wingate in U.S. Pat. No. 4,631,520 entitled Position Encoder Compensation System, wherein an essentially continuous linear function is generated based on an approximation of the distance beyond step transitions. This linear correction approach eliminates the need for a large LUT, but it also compromises the resultant performance of the measurement system by ignoring all the non-linear errors.
One general difficulty with these compensation schemes is the need to perform a calibration measurement with an independent, known good reference system. Consequently, calibration generally cannot be performed in-situ, as mentioned above, and therefore these techniques cannot compensate for errors that arise only in-situ. There is a movement in the encoder industry toward the use of so-called tape gratings, which are encoder scales manufactured on thin, flexible substrates. The largest scale errors for tape gratings are generally created upon installation, and therefore an a-priori calibration at the factory is generally not effective.
It would be desirable to have an error compensation technique for encoders that addresses the problems arising from the prior art requirement for a separate known good position measuring system when calibrating encoder scales. Additionally, an improved technique would permit in-situ re-calibration of the encoder as may be necessary or desired during operation.